Adverse effects of short-term, very-low-salt diets in subjects with risk-factor clustering

Obesity, kidney dysfunction, and inflammation: interactions in hypertension

Obesity contributes 65-75% of the risk for human primary (essential) hypertension (HT) which is a major driver of cardiovascular and kidney diseases. Kidney dysfunction, associated with increased renal sodium reabsorption and compensatory glomerular hyperfiltration, plays a key role in initiating obesity-HT and target organ injury. Mediators of kidney dysfunction and increased blood pressure include (i) elevated renal sympathetic nerve activity (RSNA); (ii) increased antinatriuretic hormones such as angiotensin II and aldosterone; (iii) relative deficiency of natriuretic hormones; (iv) renal compression by fat in and around the kidneys; and (v) activation of innate and adaptive immune cells that invade tissues throughout the body, producing inflammatory cytokines/chemokines that contribute to vascular and target organ injury, and exacerbate HT. These neurohormonal, renal, and inflammatory mechanisms of obesity-HT are interdependent. For example, excess adiposity increases the adipocyte-derived cytokine leptin which increases RSNA by stimulating the central nervous system proopiomelanocortin-melanocortin 4 receptor pathway. Excess visceral, perirenal and renal sinus fat compress the kidneys which, along with increased RSNA, contribute to renin-angiotensin-aldosterone system activation, although obesity may also activate mineralocorticoid receptors independent of aldosterone. Prolonged obesity, HT, metabolic abnormalities, and inflammation cause progressive renal injury, making HT more resistant to therapy and often requiring multiple antihypertensive drugs and concurrent treatment of dyslipidaemia, insulin resistance, diabetes, and inflammation. More effective anti-obesity drugs are needed to prevent the cascade of cardiorenal, metabolic, and immune disorders that threaten to overwhelm health care systems as obesity prevalence continues to increase.

Mechanisms of blood pressure salt sensitivity: new insights from mathematical modeling

Mathematical modeling is an important tool for understanding quantitative relationships among components of complex physiological systems and for testing competing hypotheses. We used HumMod, a large physiological model, to test hypotheses of blood pressure (BP) salt sensitivity. Systemic hemodynamics, renal, and neurohormonal responses to chronic changes in salt intake were examined during normal renal function, fixed low or high plasma angiotensin II (ANG II) levels, bilateral renal artery stenosis, increased renal sympathetic nerve activity (RSNA), and decreased nephron numbers. Simulations were run for 4 wk at salt intakes ranging from 30 to 1,000 mmol/day. Reducing functional kidney mass or fixing ANG II increased salt sensitivity. Salt sensitivity, associated with inability of ANG II to respond to changes in salt intake, occurred with smaller changes in renal blood flow but greater changes in glomerular filtration rate, renal sodium reabsorption, and total peripheral resistance (TPR). However, clamping TPR at normal or high levels had no major effect on salt sensitivity. There were no clear relationships between BP salt sensitivity and renal vascular resistance or extracellular fluid volume. Our robust mathematical model of cardiovascular, renal, endocrine, and sympathetic nervous system physiology supports the hypothesis that specific types of kidney dysfunction, associated with impaired regulation of ANG II or increased tubular sodium reabsorption, contribute to BP salt sensitivity. However, increased preglomerular resistance, increased RSNA, or inability to decrease TPR does not appear to influence salt sensitivity. This model provides a platform for testing competing concepts of long-term BP control during changes in salt intake.

The influence of high versus low sodium intake on blood pressure and haemodynamics in patients with morbid obesity

BACKGROUND

Many patients with morbid obesity (BMI > 40 kg/m) have hypertension. The complex pathophysiological abnormalities linking hypertension to obesity have not been fully clarified, but abnormal sodium handling could be an important mechanism.

METHOD

Therefore, we examined changes in body fluid compartments and haemodynamic responses (at rest and during exercise) after 5 days of a low-sodium diet (90 mmol/day) and 5 days of a high-sodium diet (250 mmol/day) in 12 morbidly obese, hypertensive patients; 12 morbidly obese, normotensive patients and 12 nonobese controls.

RESULTS

High sodium intake as compared to low sodium intake was associated with an increase in plasma volume (obese, hypertensive patients: 5 ± 4%; obese, normotensive patients: 10 ± 11%; nonobese controls: 7 ± 6%), cardiac output (CO) (obese, hypertensive patients: 17 ± 12%; obese, normotensive patients: 20 ± 16%; nonobese controls: 13 ± 14%) and stroke volume (SV) (obese, hypertensive patients: 27 ± 26%; obese, normotensive patients: 27 ± 24%; nonobese controls: 18 ± 27%) in all three groups with no differences between the groups. Despite an increase in CO during high salt intake, 24-h blood pressure (BP) was unchanged in patients and controls as a result of a reduction in total peripheral resistance (obese, hypertensive patients: -11 ± 11%; obese, normotensive patients: -10 ± 12%; nonobese controls: -5 ± 14%). Similar changes were observed during an incremental bicycle exercise test wherein CO and SV were higher, whereas mean arterial BP was unchanged at each exercise level during high sodium intake.

CONCLUSION

Despite substantial increases in CO and SV, we did not observe any significant change in BP during high sodium intake, neither in morbid obese patients nor in lean individuals.

Pathogenesis and clinical physiology of hypertension

Significant advances have been made in understanding the pathogenesis and clinical physiology of primary hypertension. This article presents an overview of the physiology of normal blood pressure control and the pathophysiologic mechanisms that predispose individuals and populations to primary hypertension. The role of genetics, environment, and the gene-environment interaction is discussed. The spectrum of changes in physiologic states that result in chronic increases of arterial blood pressure are reviewed. The nature and characteristics of feedback loops and the primary modulating systems, the central and peripheral nervous systems, and circulating and tissue hormones are reviewed. The role of the endothelium of the artery and its production of endothelin, nitric oxide, angiotensin II, as well as other vasoactive substances in response to various stimuli, is also discussed. A unifying pathway for the development of hypertension and the practical implications for the prevention and control of hypertension are discussed.

Should we restrict chloride rather than sodium?

Low-salt diets have potential for prevention and treatment of hypertension, and may also reduce risk for stroke, left ventricular hypertrophy, osteoporosis, renal stones, asthma, cataract, gastric pathology, and possibly even senile dementia. Nonetheless, the fact that salt restriction evokes certain counter-regulatory metabolic responses-- increased production of renin and angiotensin II, as well as increased sympathetic activity--that are potentially inimical to vascular health, has suggested to some observers that salt restriction might not be of unalloyed benefit, and might in fact be contraindicated in some "salt-resistant" subjects. Current epidemiology indicates that lower-salt diets tend to reduce coronary risk quite markedly in obese subjects, whereas the impact of such diets on leaner subjects (who are less likely to be salt sensitive) is equivocal--seemingly consistent with the possibility that salt restriction can exert countervailing effects on vascular health. There is considerable evidence that sodium chloride, rather than sodium per se, is responsible for the known adverse effects of dietary salt. Other non-halide sodium salts, such as sodium citrate or bicarbonate, do not raise plasma volume, increase blood pressure, boost urinary calcium loss, or promote stroke in stroke-prone rats. Nonetheless, these compounds have been shown to blunt the impact of salt restriction on renin, angiotensin II, and sympathetic activity in humans. This may rationalize limited clinical evidence that organic sodium salts can decrease blood pressure in salt-restricted hypertensives. Furthermore, organic sodium salts have an alkalinizing metabolic impact favorable to bone health. These considerations suggest that restricting dietary salt to the extent feasible, while encouraging consumption of organic sodium salts in mineral waters, soft drinks, or other nutraceuticals--preferably in conjunction with organic potassium salts and taurine--may represent a superior strategy for controlling blood pressure, promoting vascular health, and preserving bone density. Further clinical studies should determine whether a moderately salt-restricted diet supplemented with organic sodium salts has a better and more uniform impact on hypertension than salt restriction alone, while rodent studies should examine the comparative impact of these regimens on rodents prone to vascular disease.

Elevated sympathetic activity may promote insulin resistance syndrome by activating alpha-1 adrenergic receptors on adipocytes

An excess of free intracellular calcium can reduce the efficiency of insulin-mediated glucose transport by blocking the dephosphorylation of GLUT-4. Classical isoforms of protein kinase C (PKC) can interfere with insulin signalling via serine phosphorylation of IRS-1 and the insulin receptor. Parathyroid hormone (PTH), by activating phospholipase C-beta in adipocytes, can promote a sustained increase in intracellular free calcium in these cells, while also activating classical PKCs. This may rationalize the fact that insulin resistance is a typical feature of hyperparathyroidism, as well as epidemiological evidence that regular ingestion of dairy products or of ethanol--which down-regulates PTH secretion--reduces risk for insulin resistance syndrome and diabetes. Alpha-1 adrenergic receptors of adipocytes--like PTH receptors--also activate phospholipase C-beta, and thus have an effect analogous to PTH on intracellular free calcium and PKC activity in adipocytes. This suggests that, via activation of alpha-1 adrenergic receptors, increased sympathetic activity in adipose tissue may promote insulin resistance syndrome. In fact, measures which provoke increased sympathetic output--such as diuretic use and severe salt restriction--are known to compromise insulin sensitivity, whereas alpha-1 antagonist drugs, as well as drugs that act centrally to suppress sympathetic activity, typically have a favorable effect on insulin function. When insulin resistance syndrome is associated with elevated sympathetic activity--for example, in hypertensives who are obese or on diuretic therapy--measures which down-regulate sympathetic activity, or, more specifically, alpha-1 adrenergic activity, may be warranted. These include centrally acting imidazoline analogs (moxonidine, rilmenidine) and alpha-1 antagonists (doxazosin, prazosin). Taurine and high-dose pyridoxine may represent practical nutritional strategies for moderating elevated sympathetic activity, and exercise training and low-insulin-response diets may be useful in this regard as well.

A preliminary fast may potentiate response to a subsequent low-salt, low-fat vegan diet in the management of hypertension - fasting as a strategy for breaking metabolic vicious cycles

Although a salted diet appears to be a sine qua non for the development of essential hypertension, low-salt diets often have a modest or even negligible impact on the blood pressure of hypertensives; this suggests that salt, perhaps often acting in concert with other aspects of a modern, rich diet, may set in place certain metabolic vicious cycles that sustain blood pressure elevation even when dietary salt is eliminated. Therapeutic fasting is known to lower elevated blood pressure - presumably in large part because it minimizes insulin secretion - and may have the potential to break some of these vicious cycles. Goldhamer has recently reported that a regimen comprised of a water-only fast of moderate duration, followed by a transition to a low-fat, low-salt, whole-food vegan diet, achieves dramatic reductions in the blood pressure of hypertensives, such that the large majority of patients can be restored to normotensive status, in the absence of any drug therapy. Although long-term follow-up of these subjects has been sporadic, the available data suggest that these large reductions is blood pressure can be conserved in patients who remain compliant with the follow-up diet - in other words, a 'cure' for hypertension may be feasible. If a protein-sparing modified fast can be shown to be virtually as effective as a total fast for achieving these benefits, it may be possible to implement this regimen safely on an outpatient basis. The ability of therapeutic fasts to break metabolic vicious cycles may also contribute to the efficacy of fasting in the treatment of type 2 diabetes and autoimmune disorders. As a general principle, if a metabolic disorder is susceptible to prevention - but not reversal - by a specific diet, and therapeutic fasting has a temporary favorable impact on this disorder, then a more definitive therapy may consist of a therapeutic fast, followed up by the protective diet as a maintenance regimen.

New insights into the effects on blood pressure of diets low in salt and high in fruits and vegetables and low-fat dairy products

Results from the recent Dietary Approaches to Stop Hypertension (DASH)-Sodium trial provide the latest evidence concerning the effects of dietary patterns and sodium intake on blood pressure. Participants ate either the DASH diet (high in fruits, vegetables and low-fat dairy products, and reduced in saturated and total fat) or a typical US diet. Within each diet arm, participants ate higher, intermediate, and lower sodium levels, each for 30 days. The results indicated lower blood pressure with lower sodium intake for both diet groups. Although some critics would argue otherwise, these findings provide important new evidence for the value of the DASH diet and sodium reduction in controlling blood pressure.

Up-regulation of endothelial nitric oxide activity as a central strategy for prevention of ischemic stroke - just say NO to stroke!

Nitric oxide (NO) produced by the endothelium of cerebral arterioles is an important mediator of endothelium-dependent vasodilation (EDV), and also helps to prevent thrombosis and vascular remodeling. A number of risk factors for ischemic stroke are associated with impaired EDV, and this defect is usually at least partially attributable to a decrease in the production and/or stability of NO. These risk factors include hypertension, high-sodium diets, homocysteine, diabetes, visceral obesity, and aging. Conversely, many measures which may provide protection from ischemic stroke - such as ample dietary intakes of potassium, arginine, fish oil, and selenium - can have a favorable impact on EDV. Protection afforded by exercise training, estrogen replacement, statin drugs, green tea polyphenols, and cruciferous vegetables may reflect increased expression of the endothelial NO synthase. IGF-I activity stimulates endothelial NO production, and conceivably is a mediator of the protection associated with higher-protein diets in Japanese epidemiology and in hypertensive rats. These considerations prompt the conclusion that modulation of NO availability is a crucial determinant of risk for ischemic stroke. Multifactorial strategies for promoting effective cerebrovascular NO activity, complemented by measures that stabilize platelets and moderate blood viscosity, should minimize risk for ischemic stroke and help maintain vigorous cerebral perfusion into ripe old age. The possibility that such measures will also diminish risk for Alzheimer's disease, and slow the normal age-related decline in mental acuity, merits consideration. A limited amount of ecologic epidemiology suggests that both stroke and senile dementia may be extremely rare in cultures still consuming traditional unsalted whole-food diets. Other lines of evidence suggest that promotion of endothelial NO activity may decrease risk for age-related macular degeneration.

The DASH Diet, Sodium Intake and Blood Pressure Trial (DASH-sodium): rationale and design. DASH-Sodium Collaborative Research Group

The DASH Diet, Sodium Intake and Blood Pressure Trial (DASH-Sodium) is a multicenter, randomized trial comparing the effects of 3 levels of sodium intake and 2 dietary patterns on blood pressure among adults with higher than optimal blood pressure or with stage 1 hypertension (120-159/80-95 mm Hg). The 2 dietary patterns are a control diet typical of what many Americans eat, and the DASH diet, which, by comparison, emphasizes fruits, vegetables, and low-fat dairy foods, includes whole grains, poultry, fish, and nuts, and is reduced in fats, red meat, sweets, and sugar-containing beverages. The 3 sodium levels are defined as higher (typical of current US consumption), intermediate (reflecting the upper limit of current US recommendations), and lower (reflecting potentially optimal levels). Participants are randomly assigned to 1 of the 2 dietary patterns using a parallel group design and are fed each of the 3 sodium levels using a randomized crossover design. The study provides participants with all of their food during a 2-week run-in feeding period and three 30-day intervention feeding periods. Participants attend the clinic for 1 meal per day, 5 days per week, and take home food for other meals. Weight is monitored and individual energy intake adjusted to maintain baseline weight. The primary outcome is systolic blood pressure measured at the end of each intervention feeding period. Systolic blood pressure is compared across the 3 sodium levels within each diet and across the 2 diets within each sodium level. If effects previously observed in clinical trials are additive, sodium reduction and the DASH diet together may lower blood pressure to an extent not as yet demonstrated for nonpharmacologic treatment. The DASH-Sodium results will have important implications for the prevention and treatment of high blood pressure.